Make the Read Receipts dialog scrollable again (#631)

### Description

At some point between commits
[43cb531](43cb531)
and
[f4cc4d5](f4cc4d5),
in the process of switching various UI elements into unified modals, the
read receipts modal (specifically the one that opens from an individual
message's context menu) lost its ability to be scrolled through.

In this patch, I've reverted that specific component to its old form,
both to immediately return the functionality and to provide a useful
reference state for the process of creating a more polished fix.

I recommend directly patching this pull request to properly unify the
modals first, but it _should_ be functional enough on its own to be
implemented as an immediate fix if you prefer.

#### Type of change

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update

### Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings

### AI disclosure:

- [ ] Partially AI assisted (clarify which code was AI assisted and
briefly explain what it does).
- [ ] Fully AI generated (explain what all the generated code does in
moderate detail).

Author: nushea

.changeset/fix-scrollable-read-receipts.md

@@ -0,0 +1,5 @@
+---
+default: patch
+---
+
+Fix read receipt scrolling not working

src/app/components/event-readers/EventReaders.tsx

@@ -59,8 +59,8 @@ export const EventReaders = as<'div', EventReadersProps>(
           </IconButton>
         </Header>
         <Box grow="Yes">
-          <Scroll visibility="Hover" hideTrack size="300">
-            <Box className={css.Content} direction="Column">
+          <Box className={css.Content} direction="Column">
+            <Scroll visibility="Hover" hideTrack size="300">
               {latestEventReaders.map((readerId) => {
                 const name = getName(readerId);
                 const avatarMxcUrl = room.getMember(readerId)?.getMxcAvatarUrl();
@@ -107,8 +107,8 @@ export const EventReaders = as<'div', EventReadersProps>(
                   </MenuItem>
                 );
               })}
-            </Box>
-          </Scroll>
+            </Scroll>
+          </Box>
         </Box>
       </Box>
     );